Recent advances in plant genetic engineering have enabled the engineering of plants having improved characteristics or traits, such as disease resistance, insect resistance, herbicide resistance, enhanced stability or shelf-life of the ultimate consumer product obtained from the plants and improvement of the nutritional quality of the edible portions of the plant. Thus, one or more desired genes from a source different than the plant, but engineered to impart different or improved characteristics or qualities, can be incorporated into the plant's genome. New gene(s) can then be expressed in the plant cell to exhibit the desired phenotype such as a new trait or characteristic.
The proper regulatory signals must be present and be in the proper location with respect to the gene in order to obtain expression of the newly inserted gene in the plant cell. These regulatory signals may include, but are not limited to, a promoter region, a 5′ non-translated leader sequence and a 3′ transcription termination/polyadenylation sequence.
A promoter is a DNA sequence that directs cellular machinery of a plant to produce RNA from the contiguous coding sequence downstream (3′) of the promoter. The promoter region influences the rate, developmental stage, and cell type in which the RNA transcript of the gene is made. The RNA transcript is processed to produce messenger RNA (mRNA) which serves as a template for translation of the RNA sequence into the amino acid sequence of the encoded polypeptide. The 5′ non-translated leader sequence is a region of the mRNA upstream of the protein coding region that may play a role in initiation and translation of the mRNA. The 3′ transcription termination/polyadenylation signal is a non-translated region downstream of the protein coding region that functions in the plant cells to cause termination of the RNA transcript and the addition of polyadenylate nucleotides to the 3′ end of the RNA.
Expression of heterologous DNA sequences in a plant host is dependent upon the presence of an operably linked promoter that is functional within the plant host. The type of promoter sequence chosen is based on when and where within the organism expression of the heterologous DNA is desired. Where expression in specific tissues or organs is desired, tissue-preferred promoters may be used. Where gene expression in response to a stimulus is desired, inducible promoters are the regulatory element of choice. In contrast, where continuous expression is desired throughout the cells of a plant, constitutive promoters are utilized.
An inducible promoter is a promoter that is capable of directly or indirectly activating transcription of one or more DNA sequences or genes in response to an inducer. In the absence of an inducer, the DNA sequences or genes will not be transcribed, or will be transcribed at a level lower than in an induced state. The inducer can be a chemical agent, such as a metabolite, growth regulator, herbicide or phenolic compound, or a physiological stress directly imposed upon the plant such as cold, drought, heat, salt, toxins. In the case of fighting plant pests, it is also desirable to have a promoter which is induced by plant pathogens, including plant insect pests, nematodes or disease agents such as a bacterium, virus or fungus. Contact with the pathogen will induce activation of transcription, such that a pathogen-fighting protein will be produced at a time when it will be effective in defending the plant. A pathogen-induced promoter may also be used to detect contact with a pathogen, for example by expression of a detectable marker, so that the need for application of pesticides can be assessed. A plant cell containing an inducible promoter may be exposed to an inducer by externally applying the inducer to the cell or plant such as by spraying, watering, heating, or by exposure to the operative pathogen.
A constitutive promoter is a promoter that directs expression of a gene throughout the various parts of a plant and continuously throughout plant development. Examples of some constitutive promoters that are widely used for inducing the expression of heterologous genes in transgenic plants include the nopaline synthase (NOS) gene promoter, from Agrobacterium tumefaciens, (U.S. Pat. No. 5,034,322), the cauliflower mosaic virus (CaMV) 35S and 19S promoters (U.S. Pat. No. 5,352,605), those derived from any of the several actin genes, which are known to be expressed in most cells types (U.S. Pat. No. 6,002,068), and the ubiquitin promoter, which is a gene product known to accumulate in many cell types.
Additional regulatory sequences upstream and/or downstream from the core promoter sequence may be included in expression constructs of transformation vectors to bring about varying levels of expression of heterologous nucleotide sequences in a transgenic plant. Genetically altering plants through the use of genetic engineering techniques to produce plants with useful traits thus requires the availability of a variety of promoters.
In order to maximize the commercial application of transgenic plant technology, it may be useful to direct the expression of the introduced DNA in a site-specific manner. For example, it may be useful to produce toxic defensive compounds in tissues subject to pathogen attack, but not in tissues that are to be harvested and eaten by consumers. By site-directing the synthesis or storage of desirable proteins or compounds, plants can be manipulated as factories, or production systems, for a tremendous variety of compounds with commercial utility. Cell-specific promoters provide the ability to direct the synthesis of compounds, spatially and temporally, to highly specialized tissues or organs, such as roots, leaves, vascular tissues, embryos, seeds, or flowers.
Alternatively, it may be useful to inhibit expression of a native DNA sequence within a plant's tissues to achieve a desired phenotype. Such inhibition might be accomplished with transformation of the plant to comprise a tissue-preferred promoter operably linked to an antisense nucleotide sequence, such that expression of the antisense sequence produces an RNA transcript that interferes with translation of the mRNA of the native DNA sequence.
Of particular interest are promoters that are induced by plant pathogens. Pathogen infection, such as nematode infection, is a significant problem in the farming of many agriculturally significant crops. For example, soybean cyst nematode (Heterodera glycines, herein referred to as “SCN”) is a widespread pest that causes substantial damage to soybeans every year. Such damage is the result of the stunting of the soybean plant caused by the cyst nematode. The stunted plants have smaller root systems, show symptoms of mineral deficiencies in their leaves, and wilt easily. The soybean cyst nematode is believed to be responsible for yield losses in soybeans that are estimated to be in excess of $1 billion per year in North America. Other pathogenic nematodes of significance to agriculture include the potato cyst nematodes Globodera rostochiensis and Globodera pallida, which are key pests of the potato, while the beet cyst nematode Heterodera schachtii is a major problem for sugar beet growers in Europe and the United States.
The primary method of controlling nematodes has been through the application of highly toxic chemical compounds. The widespread use of chemical compounds poses many problems with regard to the environment because of the non-selectivity of the compounds and the development of insect resistance to the chemicals. Nematicides such as Aldicarb and its breakdown products are known to be highly toxic to mammals. As a result, government restrictions have been imposed on the use of these chemicals. The most widely used nematicide, methyl bromide, is scheduled to be soon retired from use, and at present, there is no promising candidate to replace this treatment. Thus, there is a great need for effective, non-chemical methods and compositions for nematode control.
Various approaches to pest control have been tried including the use of biological organisms which are typically “natural predators” of the species sought to be controlled. Such predators may include other insects, fungi, and bacteria such as Bacillus thuringiensis. Alternatively, large colonies of insect pests have been raised in captivity, sterilized and released into the environment in the hope that mating between the sterilized insects and fecund wild insects will decrease the insect population. While these approaches have had some success, they entail considerable expense and present several major difficulties. For example, it is difficult both to apply biological organisms to large areas and to cause such living organisms to remain in the treated area or on the treated plant species for an extended time. Predator insects can migrate and fungi or bacteria can be washed off of a plant or removed from a treated area by rain. Consequently, while the use of such biological controls has desirable characteristics and has met with some success, in practice these methods have not achieved the goal of controlling nematode damage to crops.
Advances in biotechnology have presented new opportunities for pest control through genetic engineering. In particular, advances in plant genetics coupled with the identification of insect growth factors and naturally-occurring plant defensive compounds or agents offer the opportunity to create transgenic crop plants capable of producing such defensive agents and thereby protect the plants against insect attack and resulting plant disease.
Additional obstacles to pest control are posed by certain pests. For example, it is known that certain nematodes, such as the soybean cyst nematode (“SCN”), can inhibit certain plant gene expression at the nematode feeding site (see Gheysen and Fenoll (2002) Annu Rev Phytopathol 40:191–219). Thus, in implementing a transgenic approach to pest control, an important factor is to increase the expression of desirable genes in response to pathogen attack. Consequently, there is a continued need for the controlled expression of genes deleterious to pests in response to plant damage.
One promising method for nematode control is the production of transgenic plants that are resistant to nematode infection and reproduction. For example, with the use of nematode-inducible promoters, plants can be genetically altered to express nematicidal proteins in response to exposure to nematodes. See, for example, U.S. Pat. No. 6,252,138, herein incorporated by reference. Alternatively, some methods use a combination of both nematode-inducible and nematode-repressible promoters to obtain nematode resistance. Thus, WO 92/21757, herein incorporated by reference, discusses the use of a two promoter system for disrupting nematode feeding sites where one nematode-inducible promoter drives expression of a toxic product that kills the plant cells at the feeding site while the other nematode-repressible promoter drives expression of a gene product that inactivates the toxic product of the first promoter under circumstances in which nematodes are not present, thereby allowing for tighter control of the deleterious effects of the toxic product on plant tissue. Similarly, with the use of proteins having a deleterious effect on nematodes, plants can be genetically altered to express such deleterious proteins in response to nematode attack.
A number of plant parasitic nematodes, such as SCN, prefer to infect root tissues. Therefore, root-specific and root-preferred promoters are very useful for developing transgenic nematode-resistant crops. The root-specific and root-preferred promoters can reduce the potential detrimental impact of the nematicidal genes on other agronomic traits.
Although these methods have potential for the treatment of nematode infection and reproduction, their effectiveness is heavily dependent upon the characteristics of the nematode-inducible or nematode-repressible promoters discussed above, as well as the deleterious properties of the proteins thereby expressed. Thus, such factors as the strength of such nematode-responsive promoters, degree of induction or repression, tissue specificity, or the like can all alter the effectiveness of these disease resistance methods. Similarly, the degree of toxicity of a gene product to nematodes, the protein's longevity after consumption by the nematode, or the like can alter the degree to which the protein is useful in controlling nematodes. Consequently, there is a continued need for the identification of root-preferred, nematode-responsive promoters and nematode-control genes for use in promoting nematode resistance.